PrimaryKey vs composite key, difference?

Question

SQLite

I can specify a key to a field, or I can create a composite key, but I don't understand the difference. That is, I can specify a key to the field, and I receive data using this key, or I can specify a composite key, but I don’t understand what a composite key means.

@Entity()
open class User(
    @PrimaryKey
    @ColumnInfo(name = "idUser")
    var id:Int,
    @ColumnInfo(name="nameUser")
    var name:String
)

@Entity(primaryKeys = ["idUser","nameUser"])
open class User(
    @ColumnInfo(name = "idUser")
    var id:Int,
    @ColumnInfo(name="nameUser")
    var name:String
)